1. Field of the Invention
This invention relates to a process for purifying tryptophan obtained from indole as its raw material according to an enzyme process. More particularly it relates to a process for obtaining tryptophan by separating unreacted indole contained in the resulting tryptophan so that the indole content in the tryptophan is restricted within 10 ppm, as well as a process for recovering the indole.
2. Description of the Prior Art
Tryptophans, particularly L-tryptophan, are one of indispensable amino acids and useful compounds used for pharmaceuticals, health foods, feeds, etc.
As processes for producing L-tryptophan, a fermentation process using glucose or the like as raw material, a semi-fermentation process using anthranilic acid or the like as raw material, and besides, a process for obtaining L-tryptophan by subjecting indole as raw material to an enzymatic condensation reaction with L-serine, a process for obtaining L-tryptophan by causing an enzyme to act on indole, pyruvic acid and ammonia, etc. have been known.
A problem raised in the process for obtaining tryptophan by subjecting indole as raw material to an enzymatic function is how to separate unreacted indole contained in the resulting enzyme reaction fluid containing tryptophan.
The reaction fluid usually contains 100 ppm or more of indole. Even when indole is contained in a slightest quantity, it gives off a peculiar bad smell. Thus it is necessary to reduce its content in the final product down to at most 10 ppm, and particularly when the product is used for feeds, down to about 3 ppm.
However, indole and tryptophan both have indole skeletone and are similar in chemical structure; hence indole has a strong adhesion to tryptophan so that it is impossible to eliminate indole down to a tolerable quantity even when a means such as washing of final crystals is employed.
Further, indole is a raw material of high cost, and hence unless it can be recovered, the cost of the product increases.
Thus, in the production of tryptophan using indole, the step of purifying tryptophan containing unreacted indole after completion of the reaction should be a process capable of completely removing other impurities and at the same time, efficiently separating and recovering indole from tryptophan.
Usually, as to the process for producing tryptophan, it has been obtained, e.g. by reacting indole with serine in the presence of an enzyme or a fungus body having a tryptophan synthase function in an aqueous medium in the vicinity of neutrality. However, the enzyme or fugus body contained in the thus obtained reaction fluid must be removed, and for removing the enzyme or fungus body, various processes have been employed such as a process of separating these substances by causing them to adsorb onto a solid substance such as active carbon, adsorptive silica gel or the like according to usual treating method for amino acid, a separation process by deposition by means of a centrifugal separator, a process for purifying amino acid by removing these impurities by means of ion exchange resins, non-polar, porous resins, or the like process.
For example, the official gazette of Japanese patent application laid-open No. Sho 56-73050/1981 discloses a process for purifying L-tryptophan by isolation with strongly acidic ion exchange resin of macroporous type.
According to these processes, however, it is impossible to separate indole from tryptophan.
Indole and tryptophan are both difficultly soluble in water; hence for example, if it is intended to dilute a slurry of tryptophan after completion of the reaction with a large quantity of water to prepare an aqueous solution of tryptophan, followed by treatment with active carbon to thereby remove unreacted indole by adsorption, then a large quantity of tryptophan is also adsorbed together with indole.
Further, if a usual gel type cation exchange resin is used as described in the above official gazette, the adsorbed aromatic or heteroaromatic amino acid is relatively difficult to elute. Thus if a cation exhange resin of porous type which is easy to elute the amino acids is used to purify tryptophan by adsorption and elution, then it is naturally considered that indole is also contained in the elute. In this case, in order to separate indole from the elute, it is necessary to add an organic solvent which is not miscible with water, such as benzene, toluene or the like, to the aqueous solution of the eluted tryptophan to extract indole into the organic solvent and separate it. According to such a process, however, troublesome operations such as solvent recovery are required and it is difficult to reduce the indole content down to a tolerable range of about 3 ppm.